For seam welding of large-diameter steel pipes, double one-layer welding is commonly employed in which the inner side is welded in one pass and then the outer side is welded in one pass by multiple-electrode submerged arc welding with three or more electrodes. FIG. 3 shows the cross-sectional shape of a double one-layer weld.
That is, submerged arc welding with two or more electrodes is applied to welding (seam welding) in the manufacture of large-diameter steel pipes, and double one-layer welding in which the inner side is welded in one pass and then the outer side is welded in one pass is employed as a highly efficient welding procedure for improved production efficiency of steel pipes (for example, Japanese Unexamined Patent Application Publication Nos. 11-138266 and 10-109171).
Although double one-layer welding is a highly efficient welding procedure, a sufficient depth of fusion must be ensured to allow an inner weld metal 4B and an outer weld metal 4A to overlap each other so that no unmelted part remains and, accordingly, it is generally carried out by applying a large current of 1,000 A or more.
However, if the heat input is increased in view of welding efficiency and inhibiting welding defects, the structure of a welded heat-affected zone becomes coarse, thus degrading weld toughness, whereas if the heat input is reduced, welding defects such as slag inclusions 13 are increased. Accordingly, some seam welding methods for large-diameter steel pipes have been proposed that are superior in preventing welding defects at low heat input.
That is, although a reduction in weld heat input is effective in increasing weld toughness, the effect of increasing toughness does not appear clearly unless the heat input is significantly reduced relative to the heat input of common seam welding, whereas a significant reduction in heat input decreases the amount of deposit metal, and the cross-sectional area of grooves must be reduced correspondingly. Therefore, without deep penetration welding, the inner weld metal and the outer weld metal do not overlap each other, thus posing the risk of a lack of penetration.
Accordingly, an increase in weld toughness requires both a significant reduction in heat input and an increase in depth of fusion, and this is extremely difficult to achieve despite various proposals that have so far been made.
As a multiple-electrode submerged arc welding method, Japanese Unexamined Patent Application Publication No. 52-82652 discloses a method for achieving a sufficient depth of fusion and a good bead shape by adjusting the bead shape using a thick wire as the last electrode while ensuring a sufficient depth of fusion using a direct-current power source for the leading electrode and using a thin wire for increased current density.
Although the method disclosed in Japanese Unexamined Patent Application Publication No. 52-82652 provides a sufficient depth of fusion and a good bead surface shape, it has been pointed out that slag does not rise sufficiently from the bottom of weld metal melted deeply by the leading electrode and may therefore remain as defects.
In addition, as a method for welding in the manufacture of large-diameter steel pipes, Japanese Unexamined Patent Application Publication No. 58-32583 discloses a method for forming a welded joint with superior weld properties while inhibiting defects and ensuring a good bead shape by subjecting the inner side to multiple-electrode submerged arc welding at low heat input and subjecting the outer side to multilayer welding by gas metal arc welding and submerged arc welding.
The welding method disclosed in Japanese Unexamined Patent Application Publication No. 58-32583, however, has extremely low efficiency and is therefore difficult to apply to actual operation because it requires low-heat-input welding for inner welding and two-layer welding for outer welding.
In addition, Japanese Unexamined Patent Application Publication No. 10-109171 mentioned above proposes a submerged arc welding method in which the depth of fusion is increased by increasing the current densities of electrodes depending on the diameters thereof, although the currents and current densities thereof are insufficient for welding of steel pipes, and it is therefore difficult to achieve both a significant reduction in heat input and an increase in depth of fusion.
In addition, Japanese Unexamined Patent Application Publication No. 2006-272377 proposes a high-current, high-current-density submerged arc welding method in which as much arc energy as possible is supplied in the thickness direction to ensure the necessary depth of fusion while inhibiting the base material from being molten in the steel width direction to avoid excessive weld heat input, thereby achieving both a reduction in heat input and deep penetration.
The technique disclosed in Japanese Unexamined Patent Application Publication No. 2006-272377, however, tends to cause welding defects such as slag inclusions in welding of steel plates having a certain thickness or more because liquid slag does not rise easily.
As described above, a practicable seam welding method for large-diameter steel pipes that is superior in preventing welding defects at low heat input has not been satisfactorily completed, and high-current, high-heat-input seam welding is currently carried out to efficiently form a defect-free, sound weld in actual operation.
As a result, ensuring good toughness in the weld metal and a welded heat-affected zone has increasingly been difficult with the recent increase in the strength of line pipes, including commercialization of API X120 line pipes.
Accordingly, in light of the above problems, it could be helpful to provide a multiple-electrode submerged arc welding method for steel that allows a reduction in weld heat input while ensuring penetration comparable to a conventional level and also inhibits welding defects.
It could also be helpful to provide a multiple-electrode submerged arc welding method for steel plates that causes few welding defects, such as slag inclusions, at low heat input.
We have already demonstrated that the use of thin wires for increased current and current density (=current/cross-sectional area of wire) is effective for achieving a sufficient depth of fusion at a weld in multiple-electrode submerged arc welding with three or more electrodes (for example, Japanese Unexamined Patent Application Publication No. 2006-272377). This is because an arc concentrated as a result of increased current and current density exerts a larger gouging force on the base material, and this effect is maximized particularly when the current and current density of the leading first and second electrodes are equal to or more than predetermined levels.
Although the increased current and current density provide deep penetration, the penetration is so deep that slag produced by a slag-metal reaction occurring in the weld metal tends to rise insufficiently, thus raising the incidence rate of welding defects.